The present invention relates to semiconductor devices, and more particularly, to semiconductor devices for producing light.
Broad spectrum light emitting semiconductor devices have conventionally been fabricated by exciting a luminous substance, such as a phosphor or a phosphor-loaded material, with a relatively narrow spectrum light source. Conventionally, in such devices a light emitting diode (LED) or laser emits light in a narrow spectrum which is shifted and/or spread in wavelength to provide a broader spectrum of light. Typically, it is desirable to provide white light from a narrow spectrum light source, such as an LED or laser. Such white light sources are described, for example, in U.S. Pat. No. 6,245,259, the disclosure of which is incorporated herein by reference as if set forth fully herein.
Typically, a white LED is formed by coating a blue emitting LED with an encapsulant material, such as a resin or silicone, that includes a luminescent material, such as a YAG:Ce phosphor, that emits yellow light in response to stimulation with blue light. Some, but not all, of the blue light that is emitted by the LED is absorbed by the phosphor, causing the phosphor to emit yellow light. The blue light emitted by the LED that is not absorbed by the phosphor combines with the yellow light emitted by the phosphor to produce light that is perceived as white by an observer. Other combinations are possible. For example, a red-emitting phosphor can be mixed with the yellow phosphor to produce light having a better color temperature and/or better color rendering properties.
One difficulty with the fabrication of white light devices is that, typically, the spectrum of light emitted by the devices is dependent on the spectrum of light emitted by the laser or LED and the amount and/or distribution of luminescent material on the device. On a single wafer or die there may be variations in the wavelength of light emitted by the particular devices on the wafer or die. Furthermore, when devices are mounted in a package, there may be variations in wavelength or intensity of emitted light from package to package. Such deviations may, for example, be the result of defects in the devices or variations as a result of manufacturing tolerances. These deviations in the output wavelength of the underlying devices may result in variations in the white light emitted by the resulting devices after application and excitation of the luminescent material. Furthermore, some methods of applying luminescent materials may result in uneven color emission from the light emitting device.